In pipeline operation, certain incidents, such as starting up a pump or closing a valve, sould produce surges in the pipeline which rapidly traverse the pipeline and, under certain circumstances, could build up to dangerous proportions and may cause pipeline damage. In some pipeline installations, conventional relief valves are installed so that when dangerously high pressures are reached the valves are opened to dump some of the fluid into a reservoir to relieve the pressure. However, the mere level of pressure is not necessarily indicative of a surge and, unless such pressure sensing devices are located at a point in the pipeline where surges are likely to be of the greatest intensity, they have to be set to operate at pressure levels which are not themselves dangerous but which may be dangerous in the event of an increasing surge. As a result, they may relieve pressures which are reached gradually and which present no threat of a surge and, as a result, a quantity of fluid is needlessly dumped to a reservoir.
A more important indicator is the rate of pressure rise and if a predetermined rate is exceeded, according to pipeline design, the surge should be relieved, even though the pressure level at the point of detection is not at a dangerous level. In some systems, differential pressure devices are provided, with instantaneous pipeline pressure being measured against a delayed pre-existing pipeline pressure so that actuation of the device would reflect increase at an excessive rate.